1. Field of the Invention
This invention relates to compositions and methods for treating subterranean formations penetrated by well bores to stimulate the production of gas therefrom. In a primary embodiment, the invention relates specifically to foam treatment fluids and associated methods for treating/ stimulating gas producing formations.
2. Description of the Prior Art
Numerous compositions and methods have been developed for treating subterranean formations penetrated by well bores to stimulate the production of hydrocarbons therefrom. The compositions and methods typically function to improve the natural drainage pattern of the formations and/or remove barriers within the formations which prevent easy passage of reservoir fluids through the formations into the well bores.
Techniques commonly employed for stimulating production of hydrocarbons from subterranean formations include "acidizing" and "fracturing". Acidizing typically involves injecting an acid or aqueous acid solution into a formation to increase the size of existing pores and to etch new flow channels therein. Fracturing typically involves injecting an aqueous fluid into a formation under pressure and at a rate such that one or more fractures in the formation are produced. The acidizing or fracturing fluid is generally recovered from the formation s that it will not damage the formation or otherwise interfere with the production of hydrocarbons therefrom.
"Foam fracturing" is a specific type of fracturing technique. It can be carried out by generating a foam on the surface and then injecting the foam under pressure into the formation. The foam can be generated on the surface by combining, for example, nitrogen gas, water and a suitable foaming surfactant. Once the desired fracturing is achieved, pressure is released at the well head causing the foam to expand and come out of the well. Foam fracturing can also be carried out by generating a foam in situ in the formation. For example, U.S. Pat. No. Re. 32,302 discloses a foam fracturing method wherein a stabilized liquid-liquid emulsion formed with liquid carbon dioxide, an aqueous liquid and a selected surfactant is injected into the formation, the carbon dioxide is heated by the formation and a high quality stabilized foam is formed therein. Upon release of pressure, the foam breaks and the carbon dioxide gas exits the formation carrying associated liquids therewith.
Unfortunately, recovery of acidizing and fracturing treatment fluids from some formations is very difficult. Treatment fluids tend to rapidly fill the pore spaces of a formation matrix when contacted therewith. Many gas formations such as certain shale gas formations and coal bed methane ("CBM") formations have a naturally low permeability. These formations are typically associated with low temperatures (e.g., less than 200.degree. F.) and low reservoir pressures (e.g., less than 1000 psi (bottom hole pressure)). High capillary forces within the pore spaces tend to hold treatment fluids therein.
Other formations are unconventional in other ways that make recovery of treatment fluids difficult. Formation characteristics such as low reservoir pressure and temperature, low porosity, low diffusivity, high fluid adsorption properties and/or inadequate wetting characteristics can prevent the treatment fluid from being expelled out of the pore spaces into the perforations, fractures and so forth that allow the fluid to flow back to the well bore.
Treatment fluids remaining in a formation, particularly a tight or otherwise unconventional formation, tend to saturate the formation and significantly affect the flow properties thereof. This kind of damage can seriously impair subsequent hydrocarbon production from the formation. For example, the permeability of some formations to gas present therein is decreased to a great extent following a stimulation treatment if substantially all of the treatment fluid is not recovered.
Also, in many formations, production of hydrocarbons is hindered by the presence of blocks of water and other connate fluids interlocked with gas in the formation. These "water blocks" also saturate the formation and impair subsequent recovery of hydrocarbons therefrom. For example, foreign fluids introduced into a gas producing formation reduce the relative permeability of the ga to liquid. Due to limited reservoir pressure and capillary forces tightly holding the fluids in the pore spaces previously occupied by the gas, blocks of the foreign fluids interlocked with gas cannot be removed from the formation by simply producing the well.